The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Virgin discovery prompts global alert

Virgin has returned nine of its Embraer 190 aircraft to service after they were grounded on the discovery of a fault with bolts that hold the plane's engine in place, which in turn prompted a worldwide alert.

The fault, which was discovered during heavy maintenance carried out on Wednesday night and Thursday, prompted the Brazilian manufacturer to issue an "alert service bulletin" to all airlines operating the Embraer 190 after being informed of the problem by Virgin.

The issue relates to bolts that hold the engine onto the engine pylon, which is attached to the plane's wing.

A full inspection of its fleet of 17 Embraer 190 aircraft found faults with nine of them, which were grounded while repairs were carried out.

"We identified the initial issue with the bolt ... we advised the manufacturer and the manufacturer has sent out a notification," a Virgin spokesman told AAP on Friday.

"These aircraft have since undergone the necessary precautionary repairs and have since returned to service," the airline said in an additional statement.

The Civil Aviation Safety Authority was also informed.

"At Virgin Australia, the safety of our aircraft is our highest priority and we have been in regular dialogue with Embraer regarding this alert," the airline said.

The Embraer 190 is slightly smaller than a Boeing 737 and is used by Virgin to service regional and metro routes across the country, including Brisbane to Sydney, as well flights to Christmas Island and the Cocos Islands.